1. Field of the Invention
The present invention relates to a set of color ink compositions for ink-jet printing and a method of forming a color ink image by use of the above-mentioned set of color ink compositions.
2. Discussion of Background
Ink-jet printing has been remarkably widely used in recent years because of the advantages that printing can be carried out at high speed, that various kinds of recording media including a sheet of plain paper are usable as image-receiving materials since the ink-jet printing is non-impact printing, and that a color image can be produced at low cost.
In order to perform excellent ink-jet printing over a long period of time, an ink composition for ink-jet printing is required to satisfy the following conditions;
(1) The ejection characteristics of the ink composition, such as the stability of the direction in which the ink composition is ejected and the stability of ink ejection are excellent. PA1 (2) The preservation stability of the ink composition is excellent. Namely, no precipitates are generated due to chemical changes of the ink composition during an extended period of storage, or while in continuous use or not in use in an ink-jet printing apparatus. As a result, the nozzles of the ink jet printing apparatus are not clogged with the ink composition, and the change of physical properties of the ink composition does not take place during the above-mentioned periods. PA1 (3) Ink images printed with the ink composition show sufficiently high contrast and clearness, free of ink spreading. PA1 (4) Printed ink images dry rapidly. PA1 (5) Ink images printed with the ink composition show excellent preservation stability in terms of the resistance to water and light. PA1 (6) With respect to the color ink composition for a full-color printer, the ink composition has excellent color tone with high purity and is capable of producing color ink images with excellent color reproduction performance. PA1 the yellow ink composition: L*=95.+-.5, a*=-6.+-.5, b*=23.+-.12, PA1 the magenta ink composition: L*=75.+-.5, a*=55.+-.25, b*=-28.+-.10, and PA1 the cyan ink composition: L*=87.+-.5, a*=35.+-.10, b*=-19.+-.8. PA1 the black ink composition: L*=45.+-.8, a*=-3.+-.5, b*=-5.+-.5. PA1 Yellow ink composition: L*=95.+-.5, a*=-6.+-.5, b*=23.+-.12, PA1 Magenta ink composition: L*=75.+-.5, a*=55.+-.25, b*=-28.+-.10, and PA1 Cyan ink composition: L*=87.+-.5, a*=35.+-.10, b*=-19.+-.8. PA1 Yellow ink composition: L*=95-100, a*=-6.+-.3, b*=23.+-.8, PA1 Magenta ink composition: L*=75.+-.3, a*=55.+-.15, b*=-28.+-.10, and PA1 Cyan ink composition: L*=87.+-.3, a*=35.+-.5, b*=-19.+-.5. PA1 Yellow ink composition: L*=97-100, a*=-6.+-.2, b*=23.+-.5, PA1 Magenta ink composition: L*=75.+-.2, a*=55.+-.10, b*=-28.+-.5, and PA1 Cyan ink composition: L*=87.+-.2, a*=35.+-.3, b*=-19.+-.3.
Many proposals are made to obtain high quality ink compositions for ink-jet printing which can meet the above-mentioned requirements. However, the fact is that there is no ink composition that can satisfy all the above-mentioned conditions.
A set of color ink compositions capable of forming color images with high water and light resistance, clearness and mutual color rendering properties is proposed in Japanese Laid-Open Patent Application 8-310116. The above-mentioned set of color ink compositions employs the combination of a yellow dye, a magenta dye, a cyan die and a black dye, each having a particular structure.
To obtain excellent color reproduction performance, the combination of C.I. Acid Yellow 23, C.I. Direct Red 227 and C.I. Acid Blue 9 is disclosed in U.S. Pat. No. 5,143,547; while the combination of C.I. Direct Yellow 86, C.I. Acid Red 52 and C.I. Acid Blue 9, in U.S. Pat. No. 5,145,519.
The above-mentioned requirements (1) to (6) mainly depend on a dye serving as a coloring agent for use in the ink composition. For instance, clear images with high contrast can be obtained by increasing the concentration of a dye in the ink composition. To increase the concentration of the dye in the ink composition, it is necessary that the solubility stability of the dye be high. However, when the solubility stability of the dye in the ink composition becomes high, the water resistance of the ink images printed by use of the above-mentioned ink composition is lowered although the preservation stability and the ejection stability of the ink composition are increased. Further, in the case where a color image is formed by ink-jet printing, excellent drying rate of the ink images is significant so as to minimize the ink spreading in a color boundary portion such as a portion where two colors are superimposed and so as to obtain a color ink image with higher resolution. In addition, the color tone of a dye for use in each color ink composition is also of great importance to improve the color reproduction of the printed ink image.
With respect to the color reproduction of a set of ink compositions, when the color gamut obtained from the chromaticness indices a* and b* of each of cyan, magenta and yellow color ink compositions becomes larger, the color reproduction becomes better. However, there is no set of color ink compositions that can produce a color image having such color measurement values as to exhibit color reproduction performance equivalent or superior to Japan Color. The above-mentioned Japan Color refers to the color obtained when Japan Color Ink SF-90 (standard process ink) is printed on Japan Paper (standard art paper) as a solid with a specific ink film thickness, which are defined by ISO-TC130 Japan committee in 1993.
The previously mentioned set of color ink compositions disclosed in Japanese Laid-Open Patent Application 8-310116 has the shortcomings that the water resistance of each dye is insufficient and the color tone and color reproduction performance are unsatisfactory.
In the color ink compositions disclosed in U.S. Pat. No. 5,143,547, C.I. Acid Yellow 23 for use in the yellow ink composition is poor in terms of the water resistance, and C.I. Direct Red 227 for use in the magenta ink composition is also unsatisfactory in terms of the water resistance and the color tone. In the combination of dyes proposed in U.S. Pat. No. 5,145,519, C.I. Direct Yellow 86 for use in the yellow ink composition has insufficient color tone.
As mentioned above, it is extremely difficult to obtain perfect combination of dyes, each of which can achieve a proper balance among the requirements for the color ink compositions.
Since the above-mentioned problems cannot be completely solved only by the dye component for use in the ink composition, other additive components such as a wetting agent and a surfactant are added to the ink composition. The wetting agent is employed to improve the ejection stability and the preservation stability of the ink composition, and the surfactant is added to the ink composition to improve the drying characteristics of the ink images printed by the ink composition.
Owing to the wetting agent, the coloring agent is stably soluble in water. Therefore, it is possible to prevent the clogging of nozzles of an ink-jet printing apparatus, which results from the precipitation of dye around the nozzles with the evaporation of water component. The more the amount of wetting agent, the better the preservation stability of the ink composition. However, when the amount of wetting agent is extremely increased, the viscosity of the ink composition is increased, and consequently, the ejection of the ink composition from the nozzles becomes difficult.
The surfactant works to decrease the surface tension of the ink composition, thereby enhancing the penetrating performance of the ink composition into an image receiving medium. The improvement of the penetrating performance of the ink composition makes it possible to accelerate the drying rate of the printed ink image. When color images are produced by an ink-jet color printer, the image quality is apt to deteriorate, for example, by the occurrence of ink spreading, in the image portions in a secondary color, such as red, green or blue obtained by superimposing two colors, even though the image quality of color images in a primary color such as yellow, magenta or cyan scarcely deteriorates. In particular, when an ink is deposited on an image receiving medium and dried without using any image fixing unit, the image quality is extremely decreased because of the ink spreading in the color boundary portions if the drying rate of the ink composition is slow. Unless a proper surfactant is selected, the ink spreading may become considerable depending on the kind of image receiving medium, and the drying characteristics may not be improved because of the mutual action of the employed surfactant and the coloring agent. Further, there occurs the problem that the surfactant itself is decomposed and such decomposition of the surfactant has an adverse effect on the image quality. In addition, even if the surface tension of the ink composition is decreased by the aid of surfactant, it is possible to prevent the ink composition from spreading on the image receiving medium after a droplet of ink composition is deposited thereon. As a result, the sharpness of the printed image is impaired.